Field of the Invention
The present invention relates to silicon carbide (SiC) crystal growth and, more particularly, to the methods of growing bulk SiC single crystals from the gaseous phase or vapor phase.
Description of Related Art
Single crystals of silicon carbide of 4H and 6H polytypes serve as lattice-matched substrates in SiC— and AlGaN-based semiconductor devices, including ultra-high-frequency AlGaN-based HEMT transistors and SiC-based devices for power switching, including Schottky diodes, field-effect transistors and bipolar transistors. Other applications include ultra-fast photoconductive switches, sensors for harsh environments, radiation detectors and others.
At present, common requirements for SiC substrates include: specific electronic properties, such as conducting n-type, conducting p-type, semi-insulating Nu-type or semi-insulating Pi-type; high crystal quality with low densities of crystal defects, such as dislocations, micropipes and carbon inclusions; large substrate diameter, such as 100 mm, 150 mm, and 200 mm. The SiC substrates must be affordable to compete with other applicable substrate materials.
Large-size commercial SiC single crystals are conventionally grown by the technique of Physical Vapor Transport (PVT). PVT growth is carried out in a sealed crucible most commonly made of graphite. In preparation for growth, the crucible is charged with polycrystalline SiC source material commonly disposed at the crucible bottom and a SiC single crystal seed commonly disposed at the crucible top. The loaded crucible is filled with inert gas to a desired pressure—generally between several and 50 Torr—and heated to the growth temperature, which is generally between 2000° C. and 2400° C. A vertical temperature gradient is created in the crucible between the SiC source and SiC seed, wherein the SiC source material temperature is higher than that of the SiC single crystal seed.
At high temperatures, the SiC source material sublimes releasing into the crucible interior a spectrum of volatile molecules, such as Si, Si2C and SiC2. Driven by the vertical temperature gradient, these species migrate to the SiC seed and condense on it causing growth of a SiC single crystal on the SiC single crystal seed. Prior art in the area of PVT growth of silicon carbide includes U.S. Pat. No. 6,805,745; U.S. Pat. No. 5,683,507; U.S. Pat. No. 5,667,587 and U.S. Pat. No. 5,746,827, all of which are incorporated herein by reference.
The conventional PVT growth process has shortcomings. Specifically, due to the incongruent character of silicon carbide sublimation, the stoichiometry of the vapor phase changes progressively during PVT growth from an initial excess of silicon towards an excess of carbon. This leads to source attrition, generation of carbon residue in the SiC source, and the appearance of carbon inclusions in the grown SiC crystal leading to crystal defects, such as dislocations and micropipes.
Several alternatives to PVT are known in the art. Among them the methods of High Temperature CVD (HTCVD), Halide CVD (HCVD), High Temperature Gas Source Method (HTGSM) and Halosilane Assisted PVT (HAPVT) disclosed in U.S. Pat. No. 6,824,611; EP0835336; EP0859879; US 2005/0255245 and US 2012/0152165, all of which are incorporated herein by reference.
All of the aforementioned methods are variations of the general method of Chemical Vapor Deposition (CVD) carried out at high temperatures of SiC sublimation growth. During growth, a gaseous silicon precursor (silane or halosilane), and a gaseous carbon precursor (a hydrocarbon, such as propane or methane), are injected into the growth crucible, where they react and form a variety of Si—C—H or Si—C—H—Cl gaseous molecules. These species migrate towards the SiC single crystal seed, adsorb on a growth interface of the SiC single crystal seed, and react in the adsorbed state causing growth of the SiC single crystal on the SiC single crystal seed. Gaseous byproducts desorb from the growth interface and leave the crucible through provided gas outlets or passages.
One of the disadvantages of the HTCVD, HCVD and HTGSM techniques stems from the use of gaseous hydrocarbons and silane, which are thermally unstable and decompose when exposed to elevated temperatures. Their thermal decomposition is accompanied by precipitation of carbon and elemental silicon leading to clogging of the gas outlets. In order to reduce this undesirable thermal decomposition, strong dilution with hydrogen, as well as high gas flow rates are employed. This, in turn, leads to low efficiency of crystallization and losses of expensive raw materials.
The method of HAPVT disclosed in U.S. Pat. No. 8,512,471, which is incorporated herein by reference, is a modified PVT technique, wherein a PVT crucible is loaded in a conventional fashion with the SiC source material (powder) and the SiC single crystal seed, and then heated to SiC sublimation growth temperatures. During growth, small amounts of a gaseous halosilane, such as SiCl4, are introduced into the crucible; optionally, hydrogen can be introduced as well. The presence of reactive gases in the crucible leads to chemical reactions between halogen, hydrogen and chemical elements present in the growth system. The benefits include removal of unwanted impurities and facilitation of SiC crystal growth.